As is well known, a lightguide fiber is an elongated glass element, which is provided with a protective coating and which has a diameter on the order of 125 microns. It comprises a cylindrical core having a predetermined refractive index and a covering layer having a different refractive index which is smaller than that of the core. Because of the difference between the refractive index of the core and that of the covering layer, a light beam entering at one end of the fiber with a sufficiently small angle of incidence with respect to the axis of the fiber is reflected inside the core and transmitted from one end of the fiber to the other.
Lightguide fiber cables have been made by first assembling up to twelve fibers side-by-side in a parallel array sandwiched between plastic layers to form a ribbon. A plurality of these ribbons are formed into a core which is wrapped with a plastic material such as TEFLON.RTM. plastic that provides thermal protection and that serves to minimize the friction between the core and an adjacent layer of the cable. Then the core and its plastic wrap are enclosed in a composite sheath comprising layers of plastic and stainless steel wires which are referred to as strength members. Typically, the layers of ribbons follow an undulated path within an inner jacket of the sheath. See U.S. Pat. No. 4,241,979 which issued on Dec. 30, 1980 in the names of P. F. Gagen and M. R. Santana.
The implementation of lightguide fiber systems entails some difficulties of a mechanical nature because of the low values of tensile strength and of ultimate elongation of the fibers. The manufacture and installation of lightguide fiber cable must be accomplished while limiting the magnitude of the stresses and deformation which can adversely affect the fibers. Tensions on the ribbons, the wires and on the cable are among the critical variables that must be controlled to avoid any undue compressive or tensile stresses and, in turn, any added fiber loss.
Production lengths of lightguide cable are typically one or two kilometers. Shipping lengths vary widely, depending on customer requirements. Production lengths are cut to a customer's specified length prior to termination. Cables are connectorized prior to shipment and lengths must be precise so that the cables can be installed without slack.
During the fabrication of lightguide fiber cable, one of the most critical parameters is the ratio of the core length to the sheath length. These two lengths are not necessarily the same. In a completed lightguide cable, relative movement can occur between the core and the sheath because of the clearance on sides of the core and the relatively low coefficient of friction between the core and its plastic wrap. The core is said to be loosely coupled to the sheath. While this may be desirable in the completed cable, it presents a problem during cable manufacture. As the cable is taken up on a reel, the back tension of each ribbon, even though only one to three ounces, causes a reduction in core length relative to the sheath. Less core per unit length of sheath is taken up on the reel. This occurs because the core does not remain at the sheath centerline, but rather moves in toward the center of each convolution on the reel away from the neutral axis of the sheath, following a shorter path than the sheath. The effect is greatest for single ribbon cores and least, but still significant, for twelve ribbon cores.
The cable ends are terminated while the cable is on the reel; therefore any discrepancy between the core length and the sheath length is not translated into undue strains in the cable on the reel. But, when the cable is payed off the reel and installed in the field, the core is moved toward the center of the sheath cross-section and since it is shorter than the sheath, the core becomes unduly strained.
The desired core-to-sheath length ratio for the cable described hereinbefore is 1.0000 but a range of 1.0000 to 1.0007 is acceptable. A wider range might be acceptable as a compromise, but any ratio below 1.0000 could be expected to reduce the life of the fiber since it would be in tension when the cable is payed out from the reel into a generally straight configuration. Ratios much above 1.0007 for the above-described cable can be expected to increase the microbending loss because the compressive load on the core would be excessive.
Relative movement between the core and the sheath could be prevented by using a cabling line of sufficient length to insure coupling of the core to the sheath by friction. For cables manufactured on such a line, the core-to-sheath ratio would be effectively 1.000. Operation of the line would be the same for all cable core constructions. It has been determined that before a first turn of a fully jacketed cable, the line length required for coupling by this method is about 1200 feet. While it is possible to build and operate, a line of such length presents many operating difficulties and increases the cost of the equipment as well as that of product. The long line approach cannot be considered a satisfactory solution to the problem.
If the ribbon and core wrap could be supplied from payouts to the formed sheath at zero back tension, it would seem that the desired core-to-sheath length ratio could be met within the presently used line. This method depends on the successful design of an apparatus to pay out the ribbon and core wrap without any back tension. While some designs have been proposed, none are considered satisfactory for all core constructions.
U.S. Pat. No. 4,153,332 addresses the problem of coupling between a single lightguide fiber and an enclosing plastic tube, a plurality of which are subsequently formed into a cable. An optical fiber is coated with an antiadhesive material before a tube is extruded thereover. The tube is elongated during its passage between a brake and a drawing capstan which permits relative sliding between the tube and the fiber. In the drawing capstan, the tube and the lightguide element are advanced in a number of turns between two sheaves of unequal diameter. Thereafter, the elongation of the tube is eliminated. As it is taken up on a collecting drum, the tube shortens and causes the fiber to become wavy so that the stretched length of the fiber is greater than the length of the tube.
It has been found that the wrapping of a coupled cable about a capstan including unequal diameter sheaves presents a problem. At each sheave in the manufacturing line for lightguide fiber cable, the plastic core wrap, being under tension, assumes the shortest path. Because the ribbon or ribbon array is captured inside the core wrap and is also under tension, it takes the shortest path. With the ribbon shorter than the sheath in a subsequent straight section, tension increases and intensifies the effect at the second sheave, if the diameter of the second sheave or takeup reel is larger than that of the first sheave.
None of the above-mentioned apparatus provide totally satisfactory solutions to the problem of substantially precisely controlling core-to-sheath length for a lightguide fiber cable. Passive line components would be acceptable if any offered both control of coupling and reasonable operating conditions. However, a cable manufacturing line that is about 1000 feet long cannot be considered reasonable from the operating point of view.